U.S. Pat. No. 6,041,131 discloses a miniature moving armature receiver that comprises a damping fluid arranged inside a magnetic gap or a coil tunnel of the receiver. The damping fluid provides improved shock protection of the receiver and/or acoustical damping of a frequency response of the receiver by damping armature movement within the magnetic gap or the coil tunnel of the receiver.
The ability to omit traditional acoustical screens or grids in a sound outlet port of the receiver to provide damping or control of the receiver frequency response is one advantage of a damping fluid. Common hearing aid design practices tend to leave the receiver's sound outlet port positioned deeply inside the hearing-aid user's ear canal where the acoustical screen is vulnerable to clogging by cerumen and/or sweat from the user's ear canal during use. Consequently, the hearing aid's sound passage becomes blocked during use and leaves the hearing aid in a partly or fully inoperative state.
A further disadvantage of acoustical screens in a hearing aid context is the imposed size requirements. The very small dimensions required for the acoustical screens render the acoustical screens difficult to manufacture with sufficient precision to provide consistent and predictable acoustical properties.
The above-mentioned prior art arrangement of damping fluid inside the magnetic gap or the coil tunnel of the receiver is associated with certain disadvantages. For example, it is difficult to introduce a correct amount of damping fluid into the magnet or coil gap to obtain the desired acoustical damping. This difficulty is caused partly by the very small dimensions of the coil gap or magnetic gap in a miniature receiver and partly by the inaccessible location of the coil gap or magnetic gap. Introducing too high or too low an amount of damping fluid will lead to a frequency response which deviates from the desired or target response. It is also difficult to ensure an even distribution of the utilized damping fluid above and below the armature so as to prevent introduction of harmonic distortion caused by asymmetrical fluid forces.
While the invention is susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and will be described in detail herein. It should be understood, however, that the invention is not intended to be limited to the particular forms disclosed. Rather, the invention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.